my xbox 360 is cracking up

[twesterm]

1. Your GTAIV disc is probably dirty or you need to clear your cache (google it).  Try clearing the cache on your 360 first,I had the same problem with Bioshock and clearing the cache fixed it.  If that doesn't help, clean your disk.  Finally, if that doesn't help, install the game if neither of those work.  If none of those work, rent the game and try another disc and see what happens.
2. Are these the same discs it sometimes reads and sometimes doesnt?
3. Do you have a lot of wireless devices that can interfere?

For 2 and 3, if you don't have a lot of wireless devices interfering, try contacting MS tech support.